The present invention relates to methods of producing field corn seed which limits outside or foreign pollen from nearby fields from fertilizing and contaminating the seed produced. In the last several years there has been concern in the grain handling industry about the ability to export corn from the U.S. Some companies, particularly in Europe, have expressed a concern that consumers may prefer grain produced without commingling of GMO (genetically modified organism) grain. Non-GMO grain is produced from traditional varieties and does not contain genes which have been introduced from other organisms via biotechnology. Since corn pollen can travel several miles and still be viable, unrealistically large isolation distances of non-GMO production fields would be required to produce corn free of contamination by GMO corn pollen. It also became apparent that: a) advanced DNA analysis techniques, such as PCR, are able to detect GMO's in food at extremely low levels, and b) it is difficult to get different countries and food companies to agree to a tolerance level higher than zero.
For decades, popcorn seed production in the corn belt area being contaminated by dent pollen has been a serious problem even with proper isolation distances. A small percentage of dent corn pollen contamination often occurs in popcorn seed fields grown under ideal conditions. This requires extra expense to separate popcorn by dent hybrid ears from the commercial crop.
Some varieties of popcorn will not set seed when pollinated by any field corn or certain other varieties of popcorn, but no difficulty is experienced in making the reciprocal cross. The genetic basic of this non-reciprocal cross-incompatibility is a multiple allelic series at the ga locus on the fourth chromosome such that Gas/Gas plants will not set seed with ga (field corn) pollen but will set seed with Ga1s or Ga1 pollen. Plants which are ga/ga will set with ga, or Gas pollen. When both ga and Gas pollen-tubes are competing in ga/Gas styles, only the Gas gametes effect fertilization. The cross-sterile allele Gas, also called GaS or Ga1s, is found both in the popcorns of commercial importance in this country and in South American popcorns.
Gametophytic alleles are comparable to the self-sterility genes found in strictly cross-fertilized plants such as Nicotiana, Oenothera, Trifolium, etc., in that there is interaction between the stylar tissue and the male gametophyte. When the female parent is homozygous recessive ga1, ga1 pollen can compete successfully against Ga1 and fertilize half of the ovules. However, if the plant used as the egg parent is heterozygous or homozygous Ga1, the ga1 pollen is a poor competitor and achieves fertilization in only 0-4% of the ovules. In the absence of competition, that is, if only ga1 pollen is used in the cross, full seed set is obtained regardless of the genotype of the female parent. Thus the incompatibility between ga1 pollen and Ga1 silk is not detectable in the absence of competing Ga1 pollen. One allele was designated Ga1s since its effect is stronger than Ga1.
The most important difference between what has been called Ga1 and Ga1s (also identified herein as Gas or GaS) is that ga1 pollen completely fails to function on styles homozygous for Ga1s even in the absence of competing pollen. Crosses of ga1 pollen on styles heterozygous for Ga1s yield a partial seed set. Less seed is set when the female parent is heterozygous Ga1sGa1 than when a heterozygous Ga1sga1 plant is used. The total amount of seed produced in such crosses varies greatly from plant to plant. Also, Ga1s may be the same as Ga plus one or more modifier genes.
While the GaS allele has been found in certain popcorns and in some exotic Central American maize population, in popcorns, the gene is also linked and otherwise associated with genes conditioning special starch characteristics, poor roots and stalks, and unacceptable levels of performance, particularly for yield. Also, efforts in converting white corn inbreds to the GaS allele (from popcorn) have resulted in lines which tended to be poorer than the original lines. Presently, this white corn material is obsolete, and its performance level is seriously deficient, particularly in root strength and yield level. In exotic corn germplasm, use of the GaS allele is made difficult due to its association with poor roots and stalks, day length sensitivity, and these exotics—as well as their derived breeding lines—are generally grossly unadapted to the US Cornbelt. The GaS gene has not been commercially used in field yellow dent or yellow flint corn due in part to: 1) the yield drag and the agronomic performance problems associated with moving the GaS gene into dent corn from popcorn or exotic maize or white corn; and 2) the difficulty in making crosses, e.g., A×B may set seed, while the reciprocal cross is sterile.
A contamination problem arose in mid-September, 2000 when a GMO corn variety, StarLink, was found to have been used in several food products. It had been approved by the EPA, USDA, and FDA for feed purposes only, and not for human food purposes. StarLink corn was found commingled in much of the U.S. corn supply. Analyzing samples from trucks, in elevators, in storage bins, and in shipments to food processors was both difficult and expensive. Quick tests using enzyme-linked immunoassay are available but they have limits of detection of about half of one percent, and they cost several dollars per sample. A much more accurate testing system is available, using PCR, however, it costs several hundred dollars and typically takes several days.
Most of the European countries have effectively put a ban on approval of grain that contains even a very low percentage of GMO contamination. Japan and Korea are in the process of defining which GMO's they will accept and which food uses they will insist on using identity-preserved non-GMO grains. Currently, U.S. baby food makers Heinz and Gerber will only use non-GMO grains. The current corn inbreds and hybrids and methods are not successful in preventing pollen contamination in cross-pollinated crops. U.S. biotech companies release, and farmers are continuing to grow, non-approved GMO varieties. This has resulted in a loss of export markets for U.S. grains. A reliable method to prevent unwanted outside GMO pollen from pollinating either a production field and/or a grower's field is needed to assist in producing non-GMO corn.